This invention relates to a cast polyurethane elastomer possessing both high resilience and high clarity.
Polyurethane elastomers are well known. See, e.g., U.S. Pat. Nos. 4,294,951, 4,555,562 and 5,599,874. In general, polyurethane elastomers can be formed by reacting an isocyanate, e.g., diphenyl methane diisocyanate (MDI), toluene diisocyanate, naphthalene diisocyanate, etc., with an organic polyol, e.g., polytetramethylene ether glycol, polyester diol, polyoxypropylene glycol, etc., and a chain extender, e.g., an aliphatic diol such as 1,4-butanediol. Additional components such as catalysts, surfactants and the like can also be added to the elastomer-forming reaction mixture in the usual amounts. The overall properties of the polyurethane will depend upon the type of polyol and isocyanate, the degree of crosslinking and cystallinity in the polyurethane, the molecular weight of the polyurethane and chemical structure of the polyurethane backbone.
U.S. Pat. No. 5,264,620 discloses liquid, low temperature stable prepolymers and quasi-prepolymers which are derived from MDI containing the 4,4'-MDI isomer and 2,4'-MDI isomer and a polytetramethylene ether glycol. To obtain the stable prepolymer, the '620 patent discloses that high amounts of the 2,4'-MDI isomer are highly desirable, e.g., in amounts up to about 90 percent.
Polyurethanes can be either thermoplastic or thermoset, i.e., cast, depending on the degree of crosslinking present. Thermoplastic polyurethanes do not have primary crosslinking while thermoset polyurethanes have a varying degree of crosslinking, depending on the functionality of the reactants.
For example, Japanese Patent Application No. 4,332,719 discloses a thermoplastic polyurethane resin free of fish eyes after the thermoplastic polyurethane resin solution is guided through a multiaxial extruder. The thermoplastic resin is obtained by employing a long chain polyol, a diisocyanate and a lower molecular weight diol having over 5 carbon atoms. The '719 application specifically discloses that by employing a diol having over 5 carbon atoms, e.g., 2-ethyl-1,3-hexanediol, 1,5-pentanediol, etc., instead of 1,4-butanediol, the thermoplastic resin will be free of fish eyes on the surface of the resulting resin after it has been sent through the extruder during the formation thereof.
The reaction to form polyurethane elastomers can be a prepolymer process wherein an excess of an isocyanate is first reacted with an organic polyol to form an intermediate isocyanate-terminated prepolymer. The intermediate prepolymer is then contacted with a diol chain extender such as the aforementioned 1,4-butanediol, which reacts with the residual terminated isocyanate groups to form the polyurethane elastomer. Alternatively, a quasi-prepolymer or one-shot process may be employed. In the quasi-prepolymer process, a portion of the polyol is first reacted with the isocyanate to form a quasi-prepolymer. The remainder of the polyol and the diol chain extender are then reacted simultaneously with the quasi-prepolymer to form the polyurethane elastomer. In the one-shot process, all of the reactants are combined simultaneously to form the polyurethane elastomer.